Role of edge dislocation and Si impurity in linking the blue luminescence and yellow luminescence in n-type GaN films

A close relationship is found between the blue and yellow luminescence bands in n-type GaN films, which are grown without intentional acceptor doping. The intensity ratio of blue luminescence to yellow luminescence (I-BL/I-YL) decreases with the increase in edge dislocation densities as demonstrated by the (102) full width at half maximum of x-ray diffraction. In addition, the I-BL/I-YL ratio decreases with the increase in Si doping. It is suggested that the edge dislocation and Si impurity play important roles in linking the blue and yellow luminescence.

Band-gap bowing and p-type doping of (Zn, Mg, Be)O wide-gap semiconductor alloys: a first-principles study

Using a first-principles band-structure method and a special quasirandom structure (SQS) approach, we systematically calculate the band gap bowing parameters and p-type doping properties of (Zn, Mg, Be)O related random ternary and quaternary alloys. We show that the bowing parameters for ZnBeO and MgBeO alloys are large and dependent on composition. This is due to the size difference and chemical mismatch between Be and Zn(Mg) atoms. We also demonstrate that adding a small amount of Be into MgO reduces the band gap indicating that the bowing parameter is larger than the band-gap difference. We select an ideal N atom with lower p atomic energy level as dopant to perform p-type doping of ZnBeO and ZnMgBeO alloys. For N doped in ZnBeO alloy, we show that the acceptor transition energies become shallower as the number of the nearest neighbor Be atoms increases. This is thought to be because of the reduction of p-d repulsion. The N-O acceptor transition energies are deep in the ZnMgBeO quaternary alloy lattice-matched to GaN substrate due to the lower valence band maximum. These decrease slightly as there are more nearest neighbor Mg atoms surrounding the N dopant. The important natural valence band alignment between ZnO, MgO, BeO, ZnBeO, and ZnMgBeO quaternary alloy is also investigated.

Temperature Sensitivity Dependence on Cavity Length in p-Type Doped and Undoped 1.3-mu m InAs-GaAs Quantum-Dot Lasers

We have fabricated 1.3-mu m InAs-GaAs quantum-dot (QD) lasers with and without p-type modulation doping and their characteristics have been investigated. We find that introducing p-type doping in active regions can improve the temperature stability of 1.3-mu m InAs-GaAs QD lasers, but it does not, increase the saturation modal gain of the QD lasers. The saturation modal gain obtained from the two types of lasers is identical (17.5 cm(-1)). Moreover, the characteristic temperature increases as cavity length increases for the two types of lasers, and it improves more significantly for the lasers with p-type doping due to their higher gain.

p-type doping of GaInNAs quaternary alloys

Using the first-principles band-structure method, we investigate the p-type doping properties and band structural parameters of the random Ga1-xInxN1-yAsy quaternary alloys. We show that the Mg-Ga substitution is a better choice than ZnGa to realize the p-type doping because of the lower transition energy level and lower formation energy. The natural valence band alignment of GaAs and GaInNAs alloys is also calculated, and we find that the valence band maximum becomes higher with the increasing in composition. Therefore, we can tailor the band offset as desired which is helpful to confine the electrons effectively in optoelectronic devices. (C) 2008 Published by Elsevier B.V.

New type of multimode-interference-type thermo-optic variable optical attenuator

A thermo-optic variable optical attenuator (VOA) based on a Mach-Zehnder interferometer and multimode-interference coupler is fabricated. Not a single-mode but a multimode waveguide is used as the input and output structures of the optical field, which greatly reduces the coupling loss of the VOA with a normal single-mode fiber. The insertion loss of the fabricated VOA is 2.52 to 2.82 dB at the wavelength of 1520 to 1570 nm. The polarization dependent loss is 0.28 to 0.45 dB at the same wavelength range. Its maximum attenuation range is up to 26.3 dB when its power consumption is 369 mW. The response frequency of the fabricated VOA is about 10 kHz. (C) 2004 Society of Photo-Optical Instrumentation Engineers.

Study on the internal reflection of the multimode-interference-type device

The internal reflection of the multimode-interference (MMI)-type device is calculated with the bidirectional beam propagation method. The calculated results indicate that the difference of the effective refractive indices between the core region and the surrounding region has a determining effect on the internal reflection of the MMI-type device. The output taper for the MMI-type combiner and splitter has a more evident effect on the internal reflection than the input taper. The internal reflection decreases with increasing the end width of the taper. For the MMI-type device with appropriate tapers, the internal reflection does not show evident degradation with the deviation of the length of the MMI region from its optimal value. (C) 2004 Society of Photo-Optical Instrumentation Engineers.

Molecular beam epitaxy growth and photoluminescence of type-II (GaAs1-xSbx/InyGa1-yAs)/GaAs bilayer quantum well

We report a systematical study on the molecular beam epitaxy growth and optical property of (GaAs1-xSbx/In-y Ga1-yAs)/GaAs bilayer quantum well (BQW) structures. It is shown that the growth temperature of the wells and the sequence of layer growth have significant influence on the interface quality and the subsequent photoluminescence (PL) spectra. Under optimized growth conditions, three high-quality (GaAsSb0.29/In0.4GaAs)/GaAs BQWs are successfully fabricated and a room temperature PL at 1314 nm is observed. The transition mechanism in the BQW is also discussed by photoluminescence and photoreflectance measurements. The results confirm experimentally a type-II band alignment of the interface between the GaAsSb and InGaAs layers.

Decoherence of two Josephson charge qubits coupled via sigma(x)sigma(x)-type coupling and exposed to sigma(x) noise

Decoherence properties of two Josephson charge qubits coupled via the sigma(x)sigma(x) type are investigated. Considering the special structure of this new design, the dissipative effects arising from the circuit impedance providing the fluxes for the qubits' superconducting quantum interference device loops coupled to the sigma(x) qubit variables are considered. The results show that the overall decoherence effects are significantly strong in this qubit design. It is found that the dissipative effects are stronger in the case of coupling to two uncorrelated baths than are found in the case of one common bath.

Investigation of Mn-implanted n-type Ge

Mn+ irons were implanted to n-type Ge(1 1 1) single crystal at room temperature with an energy of 100 keV and a dose of 3 x 10(16) cm(-2). Subsequently annealing was performed at 400degreesC for 1 h under flowing nitrogen gas. X-ray diffraction measurements show that as-implanted sample is amorphous and the structure of crystal is restored after annealing. Polycrystalline germanium is formed in annealed sample. There are no new phases found except germanium. The samples surface morphologies indicate that annealed sample has island-like feature while there is no such kind of characteristic in as-implanted sample. The elemental composition of annealed sample was analyzed by Auger electron spectroscopy. It shows that manganese ions are deeply implanted into germanium substrate and the highest manganese atomic concentration is 8% at the depth of 120 nm. The magnetic properties of samples were investigated by an alternating gradient magnetometer. The annealed sample shows ferromagnetic behavior at room temperature. (C) 2004 Elsevier B.V. All rights reserved.

Influence of thermal isolating grooves on the performance of the Mach-Zehnder interferometer-type thermo-optic variable optical attenuator

Two types of silicon-on-insulator thermo-optic variable optical attenuators (VOAs) based on a Mach-Zehnder interferometer and a multimode-interference coupler are fabricated, one with thermal isolating grooves to improve heating efficiency and the other without Comparison of optical and electrical properties, such as insertion losses, the maximum attenuation levels and the corresponding power consumptions, and the response times, is carried out between the two types of VOAs. The comparison results Indicate that use of thermal isolating grooves leads to better values for most characteristics and is an effective way to improve the performance of Mach-Zehnder interferometer-type thermo-optic devices. (c) 2005 Society of Photo-Optical Instrumentation Engineers.

A Si-based tunable narrow-band flat-top filter with multiple-step-type Fabry-Perot cavity structure

In the optical network, the quick and accurate alignment with wavelength is an important issue during the channel detection. At this point, a filter having flat-top response characteristic is an effective solution. Based on multiple-step-type Fabry-Perot cavity structure, a novel all-Si-based thermooptical tunable flat-top filter with narrow-band has been fabricated, using our patent silicon-on-reflector bonding technology. The device demonstrated a 1-dB flat-top width of 1 nm, 3-dB band of 3 nm, free spectra range of 8 nm, and the tuning range of 4.6 nm was obtained under the applied voltage of 4 V.

Photoinduced voltage shift in a three-barrier, two-well resonant tunneling structure integrated with a 1.2-mu m-thick n-type GaAs layer

By integrating a three-barrier, two-well resonant tunneling structure with a 1.2-mu m-thick, slightly doped n-GaAs layer, a photoinduced voltage shift on the order of magnitude of 100 mV in resonant current peaks has been verified at an irradiance of low light power density. The 1.2-mu m-thick, slightly doped n-GaAs layer manifests itself of playing an important role in enhancing photoelectric sensitivity. (c) 2005 Elsevier B.V. All rights reserved.

NIP a-Si : H solar cells on stanless steel with p-type nc-Si : H window layer

The successful application of boron-doped hydrogenated nanocrystalline silicon as window layer in a-Si: H nip solar cells on stainless steel foil with a thickness of 0.05 mm is reported. Open circuit voltage and fill factor of the fabricated solar cell were 0.90V and 0.70 respectively. The optical and structural properties of the p-layers have been investigated by using UV-VIS and Raman spectroscopy. It is confirmed that the p-layer is hydrogenated nanocrystalline silicon with a wide optical gap due to quantum size effect.

A new type of photoelectric response in a double barrier structure with a wide quantum well

We have calculated the photoelectric response in a specially designed double barrier structure. It has been verilied that a transfer of the internal photovoltaic effect in the quantum well to the tunnelling transport through above-barrier quasibound states of the emitter barrier may give rise to a remarkable photocurrent.

Low threshold current density 1.5 mu m strained-MQW laser by n-type modulation-doping

1.5 mu m. n-type modulation-doping InGaAsP/InGaAsP strained multiple quantum wells grown by low pressure metalorganic chemistry vapor decomposition technology is reported for the first time in the world. N-type modulation-doped lasers exhibit much lower threshold current densities than conventional lasers with undoped barrier layers. The lowest threshold current density we obtained was 1052.5 A/cm(2) for 1000 mu m long lasers with seven quantum wells. The estimated threshold current density for an infinite cavity length was 94.72A/cm(2)/well, reduced by 23.3% compared with undoped barrier lasers. The n-type modulation doping effects on the lasing characteristics in 1.5 mu m devices have been demonstrated.